This takes place in Underfell, so the Fell Bros are referred to as Papyrus and Sans.
I hope this was sad enough for you @PumaKittyCat ;w;

"Brother, where's Blue? I thought you said he would be home from training by now," Papyrus called from the kitchen. Sans shrugged. "He was supposed to call if he was staying late. I don't get why he's taking so long... hm? Carrot, are you okay?" Carrot (aka US!Pap), Blue's older brother, looked scared. "I saw a human walking around..." he mumbled. "Oh god...!" Sans realized what he was talking about, and got up. "You guys stay here! I'm backtracking to Undyne's place to look for him!"

"But I want to come! He's my brother!" Carrot said. Sans put his hands in his pockets. "Carrot, with a broken leg you won't be able to keep up. Just stay home, please." Carrot had broken his leg earlier that week during training with the Fell Bros, and Papyrus felt really bad about it, despite his would-gladly-kill-everyone-to-be-on-top impression. Carrot sighed. "Fine. But if he's not going to make it home, tell him his big bro loves him." Sans nodded, trying not to tear up at the thought of losing his friend. "I promise I will. Be back soon, guys, hopefully with Blue."

He closed the door and ran through Snowdin, planning to follow Blue's usual route to and from Undyne's house but not having to run far at all. He saw what he wished he hadn't, right next to the river — Blue laying on his side, clutching his stomach, a small pool of blood forming underneath him and a trail of human footprints next to him. "Blue!!" he called. Blue looked up as much as he could. "Hey... Fell..." he said weakly.

Sans ran over and hugged him softly. "Blue, listen — whatever you do, don't close your eyes. I'll get you back home." Blue looked down at his feet — already starting to turn to dust. "I... think it's... a bit... too late... for that..." he said, a tragic smile on his face. Sans shook his head in denial. "No! I'm not letting you die! I won't let it happen! You're my friend!" Blue rested his head on Sans' chest. "Just a friend...?"

Sans took a deep breath, not prepared for this question. "N-no, not just a friend. Listen, Blue. I've had a crush on you since day one, b-but I was... I was too fucking c-cowardly... to t-tell you..." Blue bumped his head against Sans'. "Well... you told me now... and that makes you brave..." he said. "I like you too..." Seizing his chance, Sans held Blue's head carefully and kissed him on the mouth. The smaller skeleton didn't resist, not like he could if he had the strength. "I l-love you, Fell..." Blue choked out. Sans smiled. "Love you too... h-hey, before you go... I made a promise to your brother."

"Hm?" "He told me, that if you weren't going to make it home, to tell you that he l-loves you..." Blue smiled up at the sobbing skeleton holding him. "Tell him I love him too... I love all of you, and... I'll miss you guys..." Sans held Blue close to him. "We'll miss you too..." he said. "Your bro will k-keep your scarf safe..." The small, weak skeleton smiled, tear tracks stained on his skull. "I kn-know he will... he always kept me safe... a s-scarf will be n-no problem... I was s-such a p-pain to my brother... I j-just wish I could tell him how sorry I am..."

"You weren't a pain, Blue. Not to him, not to me, not to Paps, not to anybody. You're an amazing friend and always will be even if you're gone. I love you, your brother loves you, we all love you. You are a part of this family, and we're never going to be the same without you. Don't... don't forget that..." The smaller skeleton wiped a tear from his lover's eyesocket. "I won't... p-promise you w-won't forget me...?"

"I promise... Blue..." he said, hugging him close to him. Feeling his heart beat in sync with his was comforting, but not comforting enough to stop the tears from coming as the smaller skeleton dissolved into dust right in his arms, his big blue eyes sad to leave, but happy that he got to spend his last moments with Fell. The plastic smile that had always been frozen on his face was brighter than it had ever been as nothing but dust, a teal scarf, and a sobbing hoodie-wearing skeleton was left of him.